1. Field of the Invention
The present invention relates to a method for forming a silicon oxide film (SiO.sub.2 film) on a semiconductor substrate, and to an oxide film for use in a semiconductor device, said oxide film comprising the silicon oxide film formed by said method.
2. Description of the Related Art
In the fabrication of an MOS semiconductor device, it is requisite to form an oxide film on a semiconductor substrate to provide a gate dielectric. The characteristics of a gate dielectric are greatly influenced bythe atmospheric condition under which the oxide film is formed. For instance, an oxide film comprising a silicon oxide film can be formed by dry oxidation method or wet oxidation method. The dry oxidation method comprises forming a silicon oxide film on the surface of a silicon semiconductor substrate by supplying a sufficiently dried high purity gaseous oxygen to a heated silicon semiconductor substrate. The wet oxidation method comprises forming a silicon oxide film on the surface of a silicon semiconductor substrate by supplying a high-temperature carrier gas containing water vapor to the silicon semiconductor substrate.
Pyrogenic oxidation method is a type of wet oxidation method. The pyrogenic oxidation method is highly reproducible, and it can be effected without controlling the water flow. The method comprises preparing water by combusting pure hydrogen. As compared with an oxide film formed by dry oxidation, a silicon oxide film prepared by pyrogenic oxidation is inferior in resistance against hot carriers due to the large population of electron traps attributed to water, however, is superior in dielectric breakdown characteristics and long-term reliability.
Particularly, in case a silicon oxide film is fabricated by dry oxidation in an oxidizing atmosphere comprising, for example, a compound containing a halogen element such as chlorine, e.g., HCl, Cl.sub.2, CCl.sub.4, C.sub.2 HCl.sub.3, CH.sub.2 Cl.sub.2, or C.sub.2 H.sub.3 Cl.sub.3, the following advantages are obtained:
(A) neutralization or gettering of alkali metal impurities can be effected inside the silicon oxide film; PA0 (B) layer stacking defects can be reduced; PA0 (C) time-zero dielectric breakdown (sometimes referred to simply hereinafter as "TZDB") characteristics, which is an index in the short-term evaluation of dielectric breakdown, can be improved; and PA0 (D) an improved channel mobility can be obtained. PA0 (E) fixed charge is decreased; and PA0 (F) density of states are reduced.
The dry oxidation method above using a halogen-containing atmosphere is referred to hereinafter as a "hydrochloric acid oxidation method".
On the other hand, by subjecting a silicon oxide film already formed on a substrate to a heat treatment in an inert gas atmosphere such as gaseous nitrogen or argon at a temperature in a range of from 800.degree. to 1,000.degree. C. for a duration of about 30 minutes, it is known that an interface improved in the following points is formed between a silicon semiconductor substrate and a silicon oxide film thereon:
The long-term reliability of a silicon oxide film can be evaluated by using time dependent dielectric breakdown (sometimes referred to simply hereinafter as "TDDB") characteristics as an index. The TDDB characteristics refer to a dielectric breakdown which is observed to occur after a passage of time, but not instantaneously upon applying a current stress or a voltage stress. In general, TDDB includes a region of random failure in which some of the semiconductor devices intermittently undergo breakdown, and a region of intrinsic breakdown (wear region) in which breakdown occurs rapidly on all of the semiconductor devices. Because the generation of an initial breakdown causes failure of the semiconductor devices that are put into market, it is requisite to reduce the generation of random failure to a level as low as possible. In contrast to the above, the intrinsic breakdown occurs due to the intrinsic breakdown of the silicon oxide film. Accordingly, intrinsic breakdown represents the allowable capacity limit of a silicon oxide film.
Considering the TDDB characteristics, a silicon oxide film fabricated by wet oxidation is superior to a one fabricated by dry oxidation. In other words, a silicon oxide film fabricated by wet oxidation exhibits superior long-term reliability. It is believed that --OH and --SiOH.sub.x groups in the silicon oxide film contribute to the amelioration of the TDDB characteristics.
The use of hydrochloric acid oxidation, which is a kind of dry oxidation method for the fabrication of the silicon oxide film, surely contributes to the amelioration of the TZDB characteristics to a certain extent, however, there still remains a problem that it cannot provide a film improved in TDDB characteristics. Furthermore, the process requires a difficult and complicated control of the equipments and the film deposition conditions.
As mentioned in the foregoing, the silicon oxide film fabricated by wet oxidation yields excellent TDDB characteristics. Improvements can be observed on the aforementioned fixed charges (E) and density of states (F) by further subjecting the resulting film to heat treatment in an atmosphere of an inert gas such as nitrogen or argon, however, the TZDB characteristics result lower than those obtained on a film fabricated by hydrochloric acid oxidation process.
A process for forming an insulating film is disclosed in, for example, JP-A-3-219632 (the term "JP-A-" as referred herein signifies an "unexamined published Japanese patent application"). This process comprises depositing an insulating film and then heat treating the resulting film under a reactive gas atmosphere, for instance, a gaseous atmosphere containing chlorine. According to the disclosed process, the insulating film is formed by rapid heating using an infrared-emitting lamp. That is, the insulating film is formed by a so-called dry oxidation. Thus, the TDDB characteristics of the resulting film are inferior to those of a silicon oxide film obtained by wet oxidation. Furthermore, the object of the disclosed process is to reduce the film defects attributed to dangling bonds and the like that are formed in the insulating film. In other words, the process described above does not aim to improve the TZDB and the TDDB characteristics of the film. The disclosed process further comprises forming an SiCl.sub.x -based doping layer in the vicinity of the insulating film and the substrate by performing heat treatment at 1,000.degree. X for a duration of 20 seconds to effect the so-called RTA (rapid thermal annealing).
As described in the foregoing, both of the conventional dry oxidation and wet oxidation methods fail to provide a silicon oxide film capable of achieving satisfactory characteristics concerning TZDB and TDDB.
With respect to a flash memory, which is a promising device believed to replace magnetic disks in the future, charge is injected or discharged through a floating gate to write or erase data. Although there are various methods for charge injection proposed to present, generally known are a channel hot electron injection method and a method of taking advantage of Fowler-Nordheim tunneling by applying a high electric field (e.g., 8 MV/cm or higher) to the tunnel oxide film. However, if traps should be present in the tunnel oxide film of flash memories, the charge would be trapped in the levels upon writing and/or erasing data. This causes fluctuation in the threshold voltage of the transistors constituting the flash memory, and leads to the malfunction of the flash memory. Thus, it is keenly demanded to obtain a modified tunnel oxide film, so that the trapping of charges might be suppressed upon writing and/or erasing data.
In the light of the aforementioned circumstances, the present inventors have proposed a method for forming a silicon oxide film which satisfies both of the required TZDB and TDDB characteristics in Japanese patent application Hei-6-86836 (filed on Mar. 23, 1993) and Hei 5-287494 (filed on Oct. 26, 1993). The methods for forming silicon oxide film above are certainly effective; yet, an improved method must be developed to implement a silicon oxide film having a still higher quality.